1. Field of the Invention
The present invention relates to an electroacoustic miniature transducer, having a transducer membrane, for use in a hearing aid.
2. Description of the Prior Art
In hearing aids, electroacoustic miniature transducers (microphones or earpieces) are used to convert acoustic signals into electrical signals, and to convert electrical signals into acoustic signals. For use in hearing aids, special demands are made on electroacoustic miniature transducers, in particular with respect to structural size and degree of efficiency.
From German OS 100 13 673, a micromechanical electroacoustic transducer is known that is manufactured according to the manufacturing method known from silicon semiconductor technology. The transducer has at least one supporting or carrier element, as well as a piezoelectric layer that has an electrode on its upper side and on its lower side. At least one of the electrodes extends beyond the piezoelectric layer, and is formed at least in part as an elastic membrane-and-carrier layer. In this way, a membrane-and-carrier layer having low elasticity, made of semiconductor material, is not necessary. The different layers in the known electroacoustic transducer are present due to their electrical properties (functioning as an electrode or as a piezoelectric layer) or due to their function as a carrier layer.
When used in a hearing aid, electroacoustic miniature transducers are exposed to extreme external influences. Microphones are affected above all by moisture, greases, or alcohols. In contrast, earpieces are exposed, via the auditory canal, predominantly to gaseous, liquid, or solid cerumen particles; moisture (sweat) also reaches the earpiece. In addition, dirt particles can accumulate in the sound channels of the electroacoustic miniature transducer. Together, moisture and dirt often result in a failure of the electroacoustic miniature transducer and thus of the overall hearing system.
In order to avoid contamination, electroacoustic miniature transducers, and in particular sound channels of the miniature transducers, conventionally have been partly protected using expensive coverings (screens, grids, etc.), which must be exchanged or cleaned at regular intervals. This represents a disadvantageous expense for the user.
Often, the electroacoustic miniature transducers are also used without protection, so that practice problems often occur in particular in the microphones due to the effect of air humidity, including for example loss of sensitivity or change in the transmission function due to absorption of moisture by the microphone membrane, oxidation, etc.
From the prior art, a multiplicity of surface coatings are known that can have the effect of repelling dirt, moisture, or oil. Thus, for example, from German OS 195 44 763 the use of a coating composition is known that contains polycondensates based on one or more compounds, capable of hydrolytic polycondensation, of the M elements of main groups III to V and subgroups II to IV of the periodic table. In these polycondensates, at least one organic group G, has at least two aliphatic carbon atoms, to each of which is bound at least one fluorine atom. The aliphatic carbon atoms are respectively bound, either directly via one of the carbon atoms or via a compound group A, to at least a part of central atoms M, for coating surfaces made of metal, plastics, natural materials (modified as necessary), ceramics, concrete, clay, and/or glass.
Surface coatings also are known that have a bactericidal or fungicidal effect. This effect, for example, can be based on silver ions contained in the coating, that diffuse on the surface of the layer. Such coatings are used for the sterilization of medical instruments, for example.
From United States Application Publication No. 2002/0181725, a condenser microphone is known in which the membrane is provided with a hydrophobic layer, in order to prevent the membrane from adhering to the condenser plate at the rear side of the microphone. Here, the distance between the membrane and the rear condenser plate is less than 10 μm.